Silver micropowder having excellent affinity for polar medium, and silver ink

ABSTRACT

Provided are silver nanoparticles having a good affinity (that is, dispersibility) for y-butyrolactone (C 4 H 6 O 2 ), an organic solvent which has a relatively high boiling point though having a relatively small molecular weight, and has a low viscosity and a low surface tension and which has little irritating odor. The above problems are solved by providing a silver micropowder excellent in affinity for at least y-butyrolactone, which comprises silver particles processed to adsorb at least one of 1,4-dihydroxy-2-naphthoic acid (C 11 H 8 O 4 ) and gallic acid (C 7 H 6 O 5 ) on the surfaces thereof and having an X-ray crystal particle diameter D X  of from 1 to 40 nm, preferably from 1 to 15 nm. The invention also provides a silver ink obtained by dispersing silver particles processed to adsorb an organic compound having a carboxyl group on the surfaces thereof and having an X-ray crystal particle diameter D X  of from 1 to 40 nm, preferably from 1 to 15 nm (or having a mean particle diameter D TEM , as measured through TEM microscopy, of from 3 to 40 nm, preferably from 4 to 15 nm), in y-butyrolactone.

TECHNICAL FIELD

The present invention relates to a silver micropowder excellent inaffinity for polar medium, especially for γ-butyrolactone, whichcomprises silver nanoparticles coated with an organic substance, and toa silver ink. In this description, “nanoparticle” is meant to indicate aparticle having a particle diameter of not more than 40 nm or so; and“micropowder” is meant to indicate a powder composed of nanoparticles.

BACKGROUND ART

Silver nanoparticles have high activity and can be sintered even at lowtemperatures, and have therefore been specifically noted as a patterningmaterial for poorly heat-resistant materials for quite some time. Inparticular, with the recent advancement in nanotechnologies, productionof single-nano class particles has become possible relatively in asimplified manner.

Patent Reference 1 discloses a method of mass-producing silvernanoparticles, starting from silver oxide and using an amine compound.Patent Reference 2 discloses a method of producing silver nanoparticles,comprising mixing an amine and a starting material of silver compound,and melting them. Non-Patent Reference 1 describes production of a pasteusing silver nanoparticles. Patent Reference 4 discloses a technique ofproducing silver nanoparticles having extremely good dispersibility inliquid. On the other hand, Patent Reference 3 discloses a method ofchanging a protective material for metal nanoparticles, from A to B,which comprises adding a polar solvent where an organic protectivematerial B having a functional group with a high affinity for metalparticles, such as a mercapto group or the like is dissolved therein, toa non-polar solvent where metal nanoparticles protected with an organicprotective material A exist, then stirring and mixing them.

-   Patent Reference 1: JP-A 2006-219693-   Patent Reference 2: WO04/012884-   Patent Reference 3: JP-A 2006-89786-   Patent Reference 4: JP-A 2007-39718-   Non-Patent Reference 1: Masami Nakamoto, et al., “Application of    Silver Nanoparticles to Conductive Pastes”, Chemical Industry, by    Kagaku Kogyo-Sha, October 2005, pp. 749-754

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

In general, the surfaces of silver nanoparticles are coated with anorganic protective material. The protective material plays a role ofisolating silver particles from each other in the reaction of producingthe particles. Accordingly, it is advantageous to select one having alarge molecular weight to some extent. When the molecular weight issmall, the distance between the particles may be narrow, and in somecase of wet-type synthetic reaction, sintering may go on during thereaction. If so, the particles may grow coarsely and production of amicropowder would be difficult.

On the other hand, when silver nanoparticles are utilized as an ink (inthis description, the term “ink” is not limited to a liquid one alonebut includes a pasty one produced by dispersing and incorporating silverparticles in an organic medium having a high viscosity in some degree),a suitable organic medium is preferably selected depending on theintended use. For example, γ-butyrolactone (C₄H₆O₂) may be mentioned asan organic solvent which has a relatively high boiling point thoughhaving a relatively small molecular weight, which has a low viscosityand a low surface tension and which has little irritating odor.

However, a silver micropowder having a good affinity for γ-butyrolactoneis heretofore unknown. The type of the dispersion medium applicable to asilver micropowder is significantly limited by the type of theprotective material (surfactant) that covers the surfaces of theparticles of the powder. Heretofore, due to the production constraint,the latitude in selecting the type of the protective material isextremely narrow, and the situation is that a suitable protectivematerial is extremely difficult to select in accordance with theintended application.

In consideration of the current situation as above, the presentinvention is to provide silver nanoparticles having a good affinity(that is, dispersibility) for γ-butyrolactone.

Means for Solving the Problems

In order to attain the above object, the invention provides a silvermicropowder excellent in affinity for at least γ-butyrolactone, whichcomprises silver particles processed to adsorb at least one of1,4-dihydroxy-2-naphthoic acid (C₁₁H₈O₄) and gallic acid (C₇H₆O₅) on thesurfaces thereof and having an X-ray crystal particle diameter D_(X) offrom 1 to 40 nm, preferably from 1 to 15 nm.

1,4-Dihydroxy-2-naphthoic acid and gallic acid (C₇H₈O₅) both have acarboxyl group (hydrophilic group), and are considered to be adsorbed bythe surfaces of Ag particles at the moiety of the carboxyl groupthereof.

The invention also provides a silver ink obtained by dispersing silverparticles processed to adsorb an organic compound having a carboxylgroup on the surfaces thereof and having an X-ray crystal particlediameter D_(x) of from 1 to 40 nm, preferably from 1 to 15 nm (or havinga mean particle diameter D_(TEM), as measured through TEM microscopy, offrom 3 to 40 nm, preferably from 4 to 15 nm), in γ-butyrolactone.Examples of the organic compound include the above-mentioned 1,4-dihydroxy-2-naphthoic acid and gallic acid; and one or more of thesemay be used here either singly or as combined.

ADVANTAGE OF THE INVENTION

The present invention has made it possible to provide silvernanoparticles having excellent dispersibility in γ-butyrolactone(C₄H₆O₂), an organic solvent which has a relatively high boiling pointthough having a relatively small molecular weight, and has a lowviscosity and a low surface tension and which has little irritatingodor. The silver micropowder composed of the silver nanoparticles isexpected to have various applications.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] This is a DTA curve of oleylamine-coated silver particlesbefore protective material substitution.

[FIG. 2] This is a DTA curve of silver particles processed to adsorb1,4-dihydroxy-2-naphthoic acid.

[FIG. 3] This is a DTA curve of silver particles processed to adsorbgallic acid.

[FIG. 4] This is a TEM picture of silver particles processed to adsorb1,4-dihydroxy-2-naphthoic acid.

[FIG. 5] This is a TEM picture of silver particles processed to adsorbgallic acid.

BEST MODE FOR CARRYING OUT THE INVENTION

Heretofore, in production of silver nanoparticles, the type of theprotective material (surfactant) could not be freely selected due to theproduction constraint. However, according to the method described below,the latitude in selecting the type of the protective material can bebroadened to a considerable extent, and therefore, it has becomepossible to obtain various, heretofore non-existent silvernanoparticles. With that, a novel silver ink has been realized, whichcomprises, as dispersed in γ-butyrolactone, silver particles processedto adsorb a carboxyl group-having organic compound on the surfacesthereof and having an X-ray crystal particle diameter D_(X) of from 1 to40 nm, preferably from 1 to 15 nm (or having a mean particle diameterD_(TEM), as measured through TEM microscopy, of from 3 to 40 nm,preferably from 4 to 15 nm).

It has become clarified that 1,4-dihydroxy-2-naphthoic acid and gallicacid are exemplified as the protective material substance (surfactant)to remarkably improve the dispersibility of silver nanoparticles inγ-butyrolactone. These organic compounds have a carboxyl group, and havethe property of being readily adsorbed by the surfaces of silverparticles.

The silver nanoparticles of the type can be obtained, for example,according to a process comprising “silver particles production step” and“protective material substitution step”. One typical method of theprocess is exemplified below.

<<Silver Particles Production Step>>

According to the wet process illustrated in Patent Reference 4, silvernanoparticles having a uniform particle size can be produced. Theproduction method comprises reducing a silver compound in an alcohol ora polyol using an alcohol or a polyol as the reducing agent to therebyprecipitate silver particles. However, according to the presentinventors' later studies, the inventors found a production method moresuitable to mass-production, and the present applicant disclosed it inJapanese Patent Application No. 2007-264598. This comprises dissolving asilver compound in a mixed liquid of a primary amine and 2-octanol, andkeeping it at 120 to 180° C. to thereby precipitate silver particles byutilizing the reducing power of 2-octanol. In this, the new productionmethod is exemplified in brief.

A silver compound (for example, silver nitrate) as a silver ion source,a primary amine A (having an unsaturated bond and having a molecularweight of from 200 to 400, for example, oleylamine) as a protectivematerial for precipitated silver particles, and 2-octanol as a solventcomponent and also as a reducing agent are prepared.

The primary amine A, 2-octanol and the silver compound, each taken in apredetermined amount, are mixed to produce a solution where the silvercompound is dissolved in the mixed solvent of the amine A and 2-octanol.For the liquid composition at the start of the reduction reaction, ingeneral, preferred conditions could be found within the range satisfyingthe following (i) to (iii):

-   (i) molar ratio of amine A/silver: from 1 to 10,-   (ii) molar ratio of 2-octanol/silver: from 0.5 to 15,-   (iii) molar ratio of 2-octanol/amine A: from 0.3 to 2.

Heating the liquid is begun, and the liquid is kept within a temperaturerange of from 120 to 180° C. At a temperature lower than 120° C., thereduction could hardly go on and a high reduction rate would bedifficult to stably attain. However, it is important not to heat so muchover the boiling point. The boiling point of 2-octanol is about 178° C.,and heating up to about 180° C. would be acceptable. More preferably,the heating is within a range of from 125 to 178° C. This may be carriedout under an atmospheric pressure; but preferably, the system is keptunder reflux with purging the vapor phase in the reactor with an inertgas such as nitrogen gas or the like. Silver nanoparticles could beprecipitated even though the stirring is not made so strongly; however,when the size of the reactor is large, stirring in some degree will benecessary. In a case with 2-octanol, the latitude in the stirring degreemay be broad in producing silver particles having a uniform particlesize, as compared with cases of using other alcohols (for example,isobutanol). The necessary all amount of 2-octanol may be mixed in thesystem before the start, or may be mixed during heating or afterheating. After the start of the reduction reaction, 2-octanol may besuitably added (additionally put into the system). Preferably, theretention time within the above temperature range is secured to be atleast 0.5 hours; however, it is considered that when the liquidcomposition satisfying the above (i) to (iii) is used, the reactioncould finish within about 1 hour, and even though the retention time isprolonged further more, there would be no change in the reduction rate.In general, the retention time of at most 3 hours may be enough. Whensilver particles are precipitated with the procedure of the reductionreaction, there is obtained a slurry of silver nanoparticles coated withthe amine A.

Next, a solid fraction is collected from the slurry through decantationor centrifugation. The collected solid fraction comprises mainly thesilver nanoparticles coated with the protective material of the primaryamine A.

The solid fraction is contaminated with impurities, and is preferablywashed with methanol or isopropanol.

In the manner as above, silver particles coated with the primary amine Acan be produced, having an X-ray crystal particle diameter D_(X) of from1 to 40 nm, preferably from 1 to 15 nm. As measured through observationwith a transmission electronic microscope (TEM), the mean particlediameter D_(TEM) of the particles falls within a range of from 3 to 40nm, preferably from 4 to 15 nm or so.

<<Protective Material Substitution Step>>

Next taken is an operation of changing the protective material adheringto the silver particles, from the amine A to the intended substance,organic compound B (in this, at least one of 1,4-dihydroxy-2-naphthoicacid and gallic acid). The production method for the silver particles ofthe invention is characterized by employing this step.

As the organic compound B, used is one having a carboxyl group. Acarboxyl group has the property of being readily adsorbed by silver. Theabove-mentioned amine A is an amine having an unsaturated bond andhaving a molecular weight of from 200 to 400, and its power to adhere tosilver is considered to be smaller than that of a substance having acarboxyl group. Accordingly, when a sufficient amount of molecules ofthe organic compound B exist around the surfaces of the silver particlescoated with the amine A, then the amine A is desorbed from the silversurface to produce a situation where the organic compound B could bereadily adsorbed and the substitution may go on relatively easily.

However, since the substitution goes on in a solvent, the organiccompound B must be dissolved in a solvent. As the organic compound B,selected is one having a good affinity for a polar solvent,γ-butyrolactone; and therefore, as the solvent to dissolve the organiccompound B, employed is a polar solvent. Concretely, of solvents such asisopropanol, methanol, ethanol, decalin and others, those of goodsolubility may be selected. For the organic compound B well dissolvingin isopropanol, isopropanol is advantageously selected from theviewpoint of the safety and the cost in many cases. Silver nanoparticlescoated with the amine A are made to exist in the above-mentioned polarsolvent C where the organic compound B is dissolved, and stirred at atemperature falling within a range of from 30° C. to the boiling pointof the polar solvent C. At a temperature lower than 30° C., thesubstitution could hardly go on. In case where isopropanol is used asthe polar solvent C, the stirring is taken preferably within a range offrom 35 to 80° C. The particles coated with the amine A are, in general,poorly dispersible in the polar solvent C and are readily precipitatedin the liquid, therefore requiring stirring; however, it is unnecessaryto stir them so strongly and a state where the particles could be keptfloating in the liquid may be enough.

The substitution reaction of the amine A with the organic compound Bhaving a carboxyl group is considered to occur within a relatively shortperiod of time of a few minutes or so; but from the viewpoint ofsupplying those of industrially stable quality, the substitutionreaction is preferably secure for 1 hour or more. However, even over 24hours, further substitution reaction could not go on any more, andtherefore the substitution reaction may be finished within 24 hours fromthe practical viewpoint. Preferably, the time to be taken for thesubstitution is set to fall within a range of from 1 to 7 hours.

Concretely, a liquid is previously prepared by completely dissolving anorganic compound B in a polar solvent C, and the liquid is put in onecontainer along with therein the amine A-adhering silver nanoparticlescollected as a solid fraction, and these may be stirred and mixed. Incase where the organic compound B is liquid at room temperature, “polarsolvent C where organic compound B is dissolved” as referred to in thisdescription means that the organic compound B is not separated from thepolar solvent C but the two are uniformly mixed with each other. Theequivalent amount of the organic compound B relative to the metal Ag inthe particles, B/Ag is preferably from 0.1 to 10 equivalents. In this,one carboxyl group of the organic compound B corresponds to oneequivalent relative to 1 mol of Ag. The liquid amount of the polarsolvent C may be set within a range to secure the amount enough forsilver nanoparticles to float in the liquid.

After the silver particles adsorbing the organic compound B on thesurfaces thereof are formed in the manner as above, the system isprocessed for solid-liquid separation, and preferably, for example, anoperation of “adding a washing liquid (for example, methanol orisopropanol) to the separated and collected solid fraction, thenultrasonically dispersing it and centrifuging the liquid to collect asolid fraction” is repeated a few times to thereby wash away theadhering impurities. The washed particles are silver nanoparticleshaving an X-ray crystal particle diameter D_(x) of from 1 to 40 nm,preferably from 1 to 15 nm, or having a mean particle diameter D_(TEM),as measured through TEM microscopy, of from 3 to 40 nm, preferably from4 to 15 nm, and they have a surfactant of the organic compound Badsorbed by their surfaces. The washed solid fraction is dispersed inthe intended solvent of γ-butyrolactone to give a silver ink.

EXAMPLES Example 1

According to the method mentioned below, silver particles protected witha primary amine A as a protective material, and then the protectivematerial was changed from the amine A to an organic compound B.

In this Example, oleylamine was used as the primary amine A, and1,4-dihydroxy-2-naphthoic acid was as the organic compound B, and themethod comprises the following steps.

[Silver Particles Production Step]

6009.2 g of oleylamine (special grade chemical by Kanto Chemical),2270.3 g of 2-octanol (special grade chemical by Wako Pure ChemicalIndustries), and 1495.6 g of silver nitrate crystal (special gradechemical by Kanto Chemical) were prepared.

2-Octanol, oleylamine and silver nitrate crystal were mixed to prepare asolution in which silver nitrate was completely dissolved. Thecomposition ratio was as follows:

-   -   ratio by mol of oleylamine/silver=2.5,    -   ratio by mol of alcohol/silver=2.0,    -   ratio by mol of alcohol/oleylamine=2.0/2.5=0.8.

10 L of the liquid of the above composition was prepared, transferredinto a container equipped with a reflux condenser, put in an oil bath,and with stirring at 100 rpm with a propeller, this was heated up to120° C. at a heating speed of 1.0° C./min, and then up to 140° C. at aheating speed of 0.5° C./min. Next, with keeping the above stirringstate, this was left at 140° C. for 1 hour. During this, the vapor phasein the container was purged with nitrogen gas at a flow rate of 500mL/min. Next, the heating was stopped, and this was cooled.

After the reaction, the slurry was kept static for 3 days, and then thesupernatant was removed. In this step, the amount of the supernatant tobe removed was so controlled that the reduced silver could be 20% bymass to the whole slurry. After the supernatant removal, 1700 g ofisopropanol was added to 500 g of the slurry, then kept stirred at 400rpm with a propeller for 1 hour, and thereafter centrifuged to collect asolid fraction containing silver particles. In the thus-washed solidfraction, silver particles coated with amine A (oleylamine) exist.

Analysis of the unwashed slurry taken separately confirmed the existenceof about 1 mol of metal Ag in 500 g of the unwashed slurry.

Apart from the above, the washed solid fraction produced under the samecondition as above was sampled in a small amount, and the sample wasanalyzed in the manner mentioned below to determine the X-ray crystalparticle diameter D_(x). As a result, D_(x) of the silver micropowderbefore substitution was confirmed to be about 7 nm. In addition, themean particle diameter D_(TEM) was determined in the manner mentionedbelow. As a result, D_(TEM) of the silver micropowder beforesubstitution was confirmed to be about 8 nm. From the washed solidfraction, as produced under the same condition as above, theoleylamine-coated silver micropowder before substitution was collectedand analyzed through TG-DTA at a heating speed of 10° C./min. The DTAcurve is shown in FIG. 1. In FIG. 1, the large mountain appearingbetween 200 and 300° C. and the peak appearing between 300 and 330° C.are considered to be derived from the amine A, oleylamine.

<Determination of X-ray Crystal Particle Diameter D_(x)>

The solid fraction sample of silver particles was applied onto a glasscell, set in an X-ray diffractiometer, and based on the diffraction peakof the Ag (111) plane, the X-ray crystal particle diameter D_(x) wascomputed according to the Scherrer's formula of the following formula(I). For the X-ray, used was Cu—Kα.D _(x) =K·λ/(β·cos θ)  (1)In this, K is the Scherrer's constant, and is 0.94. λ is the X-raywavelength of the Cu—Kα ray; β is the half-value width of theabove-mentioned diffraction peak; and θ is the Bragg angle of thediffraction line.<Determination of Mean Particle Diameter D_(TEM)>

The silver particle dispersion was observed with a transmissionelectronic microscope (TEM), through which independent, non-overlapping300 silver particles were analyzed for the particle diameter thereof,and the data were averaged to compute the mean particle diameter.

[Protective Material Substitution Step]

1,4-Dihydroxy-2-naphthoic acid (special grade chemical by Wako PureChemical Industries, having a molecular weight of 204.18) was preparedas the organic compound B, and isopropanol (special grade chemical byWako Pure Chemical Industries, having a molecular weight of 60.1) was asthe polar solvent C.

56.8 g of 1,4-dihydroxy-2-naphthoic acid and 400 g of isopropanol weremixed and kept at a liquid temperature of 40° C. whereby1,4-dihydroxy-2-naphthoic acid was completely dissolved in isopropanol.The above-mentioned washed solid fraction with amine A(oleylamine)-coated silver particles existing therein (containing Ag inan amount of about 1 mol (about 100 g)) was added to 456.8 g of theabove liquid, and stirred at 400 rpm with a propeller. With keeping thestirring state, this was left at 40° C. for 5 hours. In this case, theamount of the organic compound B to be added to the system was socontrolled that the amount of the organic compound B relative to Agcould be 0.3 equivalents.

The resulting slurry was centrifuged at 3000 rpm×5 min for solid-liquidseparation. Next, this was processed according to an operation of“adding 889.7 g of methanol (about 30 equivalents relative to silver) tothe solid fraction, then washing it at 400 rpm for 30 minutes, andcollecting the solid fraction through centrifugation” repeated twice,thereby giving a silver micropowder sample where the protective materialwas substituted with 1,4-dihydroxy-2-naphthoic acid.

The sample was analyzed through TG-DTA according to the above-mentionedmethod. Its DTA curve is shown in FIG. 2. By comparison between FIG. 1(before substitution) and FIG. 2 (after substitution), it is consideredthat almost all the amine A (oleylamine) of the protective material wasdesorbed and was substituted with the organic compound B(1,4-dihydroxy-2-naphthoic acid). FIG. 4 shows a TEM picture of silverparticles that have adsorbed 1,4-dihydroxy-2-naphthoic acid.

The sample was analyzed in the same manner as above to determine theX-ray crystal particle diameter D_(x) and the mean particle diameterD_(TEM) thereof, and D_(x) was 7.57 nm and D_(TEM) was 8.45 nm.

Regarding the particle diameter of each particle used in computingD_(TEM), the minimum value D_(min) was 6.10 nm, and the maximum valueD_(max) was 13.44 nm. The standard deviation of the particle diameter isrepresented by σ_(D)p, and the value of “σ_(D)/D_(TEM)×100” is called aCV value. The CV value of the silver micropowder was 14.2%. It may besaid that silver particles having a smaller CV value could have a moreuniform particle diameter. In application to silver ink, the CV value ispreferably at most 40%; and those having a CV value of at most 15% havean extremely uniform particle diameter, and are extremely favorable forapplication to various microwiring.

Next, for evaluating the affinity for γ-butyrolactone, the sample wastested in a dispersibility test. 10 g of γ-butyrolactone was put into abeaker, 0.5 g of the sample was put into the beaker and lightly stirred,and then this was ultrasonicated for 10 minutes and was therebyuniformly dispersed, then kept static at room temperature for 168 hours,and thereafter the liquid was visually checked as to whether or not theliquid became cloudy or as to whether or not the liquid becameprecipitated or flocculated, whereby the affinity of the sample wasevaluated. The evaluation standards are as follows: The case in whichparticles completely precipitated in 168 hours and the supernatantbecame transparent is confirmed poor in the affinity; and the case inwhich particles did not precipitate in 168 hours and the supernatant waskept cloudy is confirmed rich in the affinity. As a result, gooddispersibility of the sample was confirmed. Specifically, it wasconfirmed that the silver nanoparticles processed to adsorb1,4-dihydroxy-2-naphthoic acid as the protective material are readilydispersible in γ-butyrolactone and have an excellent affinity for it.

Example 2

This is the same experiment as in Example 1 except that the organiccompound B was changed to gallic acid (special grade chemical by WakoPure Chemical Industries, having a molecular weight of 170.1).

Concretely, in the protective material substitution step, 78.83 g ofgallic acid and 400 g of isopropanol were mixed and kept at a liquidtemperature of 40° C. whereby gallic acid was completely dissolved inisopropanol. The above-mentioned washed solid fraction with amine A(oleylamine)-coated silver particles existing therein (containing Ag inan amount of about 1 mol (about 100 g)) was added to 478.83 g of theabove liquid, and stirred at 400 rpm with a propeller. With keeping thestirring state, this was left at 40° C. for 5 hours. In this case, theamount of the organic compound B to be added to the system was socontrolled that the amount of the organic compound B relative to Agcould be 0.5 equivalents.

The DTA curve of the obtained sample is shown in FIG. 3. By comparisonbetween FIG. 1 (before substitution) and FIG. 3 (after substitution), itis considered that almost all the amine A (oleylamine) of the protectivematerial was desorbed and was substituted with the organic compound B(gallic acid). FIG. 5 shows a TEM picture of silver particles that haveadsorbed gallic acid.

D_(X) of the sample was 6.58 nm and D_(TEM) thereof was 8.54 nm.Regarding the particle diameter of each particle used in computingD_(TEM) the minimum value D_(min) was 3.99 nm, and the maximum valueD_(max) was 13.73 nm; and the CV value of the silver micropowder was19.8%.

As a result of the dispersibility test in γ-butyrolactone, the samplewas confirmed to have a good dispersibility. Specifically, it wasconfirmed that the silver nanoparticles processed to adsorb gallic acidas the protective material are readily dispersible in γ-butyrolactoneand have an excellent affinity for it.

The invention claimed is:
 1. A silver micropowder having affinity for atleast γ-butyrolactone, which comprises silver particles processed toabsorb 1,4-dihydroxy-2-naphthoic acid on the surfaces thereof and havinga crystal diameter Dx of from 1 to 40 nm determined by X-ray diffractionmethod.
 2. A silver ink obtained by dispersing silver particlesprocessed to absorb an organic compound having a carboxyl group on thesurfaces thereof and having a crystal diameter Dx of from 1 to 40 nmdetermined by X-ray diffraction method, in γ-butyrolactone.
 3. Thesilver ink as claimed in claim 2, wherein the organic compound is atleast one of 1,4-dihydroxy-2-naphthoic acid and gallic acid.